Background noise expressed at different levels occurs for MRT images as a function of the recording sequence. For example, for diffusion-weighted MRT images that have been recorded with the aid of an EPI (Echo Planar Imaging) sequence the background tends to be very noisy. Such images are often used for further calculations in order to display clinically relevant information more effectively. Thus, a number of diffusion-weighted images of the same slice plane are used with a different level of diffusion weighting to calculate an ADC (Apparent Diffusion Coefficient) image by using a nonlinear fit to match the image data of at least two images having a different diffusion weighting.
In this method, the background noise of the diffusion-weighted images is intensified and generates an interfering noise in the ADC image. So far, the problem has been solved by generating a background mask that masks the noisy background areas. In this case, all the pixels whose intensity in the image with the diffusion weighting of level zero is below a specific threshold value form the background mask. This background mask sets the intensity of the corresponding pixels to zero in ADC images of the same slice plane. The threshold value is set manually as a rule.
In the case of the SIEMENS MAGNETOM Harmony MRT apparatus, for example, the threshold value to be used is set manually before beginning a measurement. The success of the method depends in this case on the experience of the user who carries out the measurement and determines the threshold value. A wrongly set threshold value can lead to a loss of medically important information. The measurement must be repeated, if appropriate, and this entails higher costs and a relatively larger outlay on time.
DE 103 56 275 A1 describes a method for automatically segmenting phase-coded flow images in MRT. Here, at least one phase image of an area flowed through by blood is measured with the aid of the MRT and areas flowed through are thereupon automatically segmented in the phase image. On the basis of a histogram that is calculated from the phase image, inter alia, the segmenting can be performed by stipulating a threshold value in the histogram. It is likewise possible to segment areas flowed through by using a seed growth algorithm that scans the existing flow regions from inside out starting from a nucleus in the interior of the area flowed through.
US 2004/0254447 A1 discloses a method of background suppression for time-resolved magnetic resonance angiographies. In this method, an orthogonality image that is masked in a further method step is calculated from a series of temporally resolved MRT images. To this end, a histogram of the orthogonality image is processed and, inter alia, fitted with the aid of a Gaussian curve, and a threshold value is calculated on the basis thereof.
DE 101 22 874 A1 discloses a method for extracting spin collectives with different chemical shift from phase-coded individual images by taking account of field inhomogeneities. In this method, a threshold value is stipulated on the basis of an automatic noise level estimate.